The subject matter disclosed herein relates to a method of determining the position of a motor and, more specifically, to a method for compensating accumulated position error in a motor drive for a synchronous motor.
As is known in the art, synchronous motors are designed such that the rotor rotates at the same speed as the rotating magnetic field established in the stator. In a permanent magnet (PM) synchronous motor, magnets are embedded in or mounted to the rotor and establish the magnetic field in the rotor. As an alternating current (AC) voltage is provided to the stator, a magnetic field is established in the stator. The magnitude of torque produced by the PM motor is a function of the angular alignment of the magnetic fields present in the stator and rotor. Because the rotor magnetic field is always present, it is desirable to know the angular position of the rotor and the relationship of the rotor magnetic field to the angular position of the rotor such that the AC voltage may be applied to the stator at the correct electrical angle. An encoder may be mounted to the PM motor to provide a measurement of the angular position.
One application in which PM synchronous motors are being used is to control the operation of an elevator cab. Historically, either direct current (DC) motors or AC induction motors have been the primary motor utilized to control the elevator cab. These motors are mounted in a machine room, often on the roof of a building, above the elevator shaft. The motors are connected via a gearbox to a sheave around which the ropes to the elevator cab are run. PM motors however, provide greater torque density, allowing a motor physically smaller than the DC or AC motor to control an elevator cab of comparable capacity. In addition to providing greater torque density, PM motors for controlling elevator cabs have been designed to provide a smaller footprint. These PM motors may include a high pole count, radial flux construction, and external rotors. The motors are typically larger in radius than axial length and may further include a sheave mounted to the rotor providing direct drive of the elevator cab. The improved torque density and physical construction may also allow the PM motor to be mounted in the elevator shaft eliminating the machine room which, in turn reduces expense and improves the aesthetics of a building.
However, the physical construction of the PM motor can impact the ability to mount an encoder to the motor. Because of the external rotor, the PM motor may not include a central rotating member. If the PM motor is, for example, a dual-rotor motor including a central rotating member, it may nevertheless be undesirable to include a shaft extending axially to which an encoder may be mounted. The shaft will increase the axial length of the PM motor, which, for a shaft-mounted PM motor, protrudes further into the elevator shaft. Consequently, an encoder that includes a friction wheel, which is mounted radially from the PM motor, configured to engage a surface of the external rotor may be mounted to the PM motor.
However, an encoder utilizing a friction wheel to engage a rotor has various disadvantages. Rather than being driven directly by the rotor, the encoder is driven by the friction wheel. As a result, the encoder generates an angular position signal that corresponds to the angular position of the friction wheel. The friction wheel has a diameter that may be several times smaller than the diameter of the rotating surface which it engages. In order to determine the angular position of the rotor, the ratio between the diameter of the friction wheel and the rotating surface must be used. Error in the value of the angular position for the rotor may be accumulated as a function of the level of precision used for the ratio. Further, the friction wheel is subject to slippage against the rotating surface, resulting in further position error between the angular position signal generated by the encoder and the angular position of the rotor. Because the PM motor may have a high pole count, a small amount of error in determining the angular position of the rotor may result in a substantial error in the electrical angle of the voltage applied to the stator.
Recently, methods of compensating the angular position to correct accumulated position errors have been developed. These methods utilize sensorless techniques to determine an estimated angular position of the motor. The sensorless techniques use either commanded or measured values of electrical signals, such as the voltage or current provided to the motor, to determine the angular position of the motor. The angle of the electrical signal is extracted twin the commanded or measured value and, based on the properties of the motor and knowledge of how the motor would respond to the electrical signal, the angular position of the motor is determined. However, these sensorless techniques often rely on electrical signals that are either not well defined or subject to electrical noise at low speeds. As a result, they are unable to compensate accumulated position error below, for example, one-third of rated speed. Thus, it would be desirable to provide a system that is able to compensate accumulated position error across the full operating range of the motor.